1. Field of the Invention
This invention relates to an improved method of reconditioning and reclaiming transformer insulating oil. For the purposes of this application, reconditioning means the removal of moisture and solid materials by mechanical means and the removal of moisture and gas by subjecting the oil to high levels of vacuum. Reclaiming is the removal of acidic compounds and other products of oxidation and colloidal contaminants by treatment with an adsorbent.
2. The Prior Art
Both reconditioning and reclaiming of transformer oil are practiced in the art. As described in the Institute of Electrical and Electronic Engineers Guide for Acceptance and Maintenance of Insulating Oil in Equipment (ANSI/IEEE C57.106-1977), the reconditioning step when contacting the oil with filter media and/or when performing the mechanical separation of oil and water by centrifuging must be accomplished at low temperature to avoid a high level of dissolved water in the oil, since the saturation amount increases rapidly with temperature. The Guide further describes that in reclaiming, the oil is usually treated by contact with Fuller's earth, an adsorbent clay material which is effective in removing the oxidation products and acids. It also states that irrespective of the type of clay treating process used for the reclaiming, the oil should be put through some device for removing free water before it contacts the clay in order to prevent water (free) from wetting the clay. Water (free) is said to cause at least partial and possibly complete blocking of the clay, thus rendering it ineffective to remove the acids and oxidation products which are detrimental to the oil.
The above Guide also describes vacuum dehydrators (degasifiers) as an efficient means of reducing water and gas in the oil to a very low value. There is considerable other literature which describes this vacuum dehydrator step. Typical references to this step are U.S. Pat. No. 3,675,395 (July 11, 1972) by Baranowski and U.S. Pat. No. 3,339,346, (Sept. 5, 1967) by Buchanan, both of which describe an apparatus for heating, filtering/coalescing and vacuum degasifying of transformer oil, and an article by Robert Schenck in January 1983 Transmission and Distribution magazine describing apparatus for heating, filtering and vacuum degasifying of transformer oils. This latter reference discusses use of a cold trap operating at -85.degree. F. to obtain adequate vacuum to sufficiently reduce the water content to meet specifications. This cold trap is refrigerated with a cascade refrigeration unit to attain the very low temperature. Another reference is Pennwalt Corporation Bulletin 950 (981-SM-1P) and Data Sheet 910D (583-5M-1P), further describing the type of units discussed in the Schenck article. The references reveal that the transformer oil to be degasified, should be at a temperature of 40.degree.-45.degree. F. above existing oil temperature or in one case (U.S. Pat. No. 3,339,346) a minimum of 122.degree. F. The industry range in practice seems to be about 120.degree.-160.degree. F.
Thus, from the standpoint of dealing with the water in transformer oils, the mechanical separation techniques want low temperatures and the degasifying step wants significantly higher temperatures. Both conditions are consistent with the mass transfer principals involved in the reconditioning process. However, in the reclaiming process, i.e., the removal of certain chemical compounds from the contaminated oil, the mass transfer principles involved dictate that such high viscosity liquids as transformer oils should be heated to a temperature that reduces the viscosity to permit a practical rate of mass transfer of the compounds from the oil to the adsorbent (Fuller's earth). This feature is mentioned in the Pennwalt Bulletin 950.
The tests for measuring the properties of transformer oil that affect its performance as an insulator and coolant are mostly those established under the American Society for Testing Materials (ASTM). These are numerous, but those particularly relevant are the measures of insulating quality which are Dielectric Breakdown Strength (indirect measure) and Water Content (direct measure); and the measures of contaminating chemical compounds formed from the deterioration of the oil, Acid Number and Interfacial Tension. The Acid Number is a direct measure of corrosive compounds in the oil and the Interfacial Tension is an indirect measure of sludge formation which adversely affects insulating and coolant properties.
From experience, specifications have been developed that state the values of the results of these ASTM test methods which insure the user of a satisfactory oil. These values have been specified for new and used or "in service" oil. The ASTM (Spec. D-3487) gives a set of values for new oil. The IEEE, as cited earlier, reports values for new and used oil for service at several voltage levels. Manufacturers of transformer equipment may specify the ASTM limit values (D-3487) as satisfactory for low and medium voltage transformers. The manufacturers usually have more stringent limits for high voltage transformers.